A study was made on the clinical and biochemical features of siblings of patients with hyperchylomicronemia and its inherited relationship. It was not a case of the classical type of familial LPL deficiency, but of familial apolipoprotein C-II deficiency. The first patient with apolipoprotein C-II deficiency was reported by Breckenridge et al. and our patients provide the basis for the second report of this new disease. Our observations in this study strongly suggest that familial apolipoprotein C-II deficiency is transmitted by an autosomal recessive mode of inheritance and heterozygotes of this disorder have no abnormalities of plasma lipid and lipoproteins in spite of the reduced plasma apolipoprotein C-II.